This invention relates to a process for treating a stack of dielectric film to render piezoelectric-sensitive (PES) film in such stack piezoelectric, and more particularly, to a high dc field intensity technique for the production of piezoelectric film which technique minimizes catastrophic breakdown of the film during processing.
Films of dielectric materials have been observed to be capable of acquiring a static electric charge upon being poled (i.e., being exposed to a dc field) which static charge is quasi-permanent, the degree of permanency being dependent largely upon the chemical constitution of the film. Such a charge, due largely to trapped positive and negative carriers positioned at or near the two surfaces of the dielectric respectively, produces what is known as an electret. When the opposing surfaces of an electret are not covered by metal electrodes, the structure possesses an electrostatic field and is, in a sense, the electrostatic analogue of the permanent magnet.
In addition to being capable of acquiring a static electric charge (and, as a result acquiring an electrostatic field), a certain limited class of film also acquires an ordered internal molecular orientation when exposed to a dc field so that even when the static (or electret characterizing) charges are dissipated, the film thereafter possesses the property of being able to generate an electric current between the electrically connected opposed surfaces by changing the pressure imposed on the opposed surfaces. This property is known as the piezoelectric effect and some such film (e.g., film made from polyvinylidene fluoride (PVDF)) also possesses a pyroelectric effect (i.e., the property of producing electron flow by changing the temperature of the film). Also, when a voltage is applied to the opposite surfaces of a piezoelectric film, a reverse piezoelectric effect occurs, i.e., the film deforms in proportion to the voltage applied to it.
Unfortunately, the term "electret" has been broadly used in the art to indicate any film that has been poled whether or not there has been molecular orientation; obviously such broad application of the term is misleading. The confusion is compounded by the fact that piezo- and pyroelectric films are capable, during exposure to a dc field, of acquiring static "electret-type" charges along with the internal molecular orientation. The present invention is directed to the production of a piezoelectric film which may also be pyroelectric.